Demystifying charge-compensation mechanisms and Oxygen dimerization in Li-rich Li2NiO3 cathodes

Li-rich cathodes are gaining popularity for Li-ion batteries due to their higher capacity compared to standard layered cathodes. However, the redox mechanisms in these materials are still not clear, nor is the origin of the extra capacity observed experimentally. We investigate the elusive charge- compensation mechanisms and their impact on potential oxygen-dimer formation in a recently synthesised Li-rich cathode, Li2NiO3. Using state-of-the-art ab initio dynamical mean-field theory, we show that the excess capacity in Li2NiO3 comes from a combined Ni and O redox, unlike its layered counterpart LiNiO2, where O redox predominates. Moreover, we demonstrate O dimer formation via a plot of the electron localisation function for the first time, and attribute this formation to the higher oxidation state of O, even in the pristine material. Finally, we show that Li migration to the interlayer tetrahedral sites at the end of charge is potentially unlikely due to the end configuration being higher in energy and the stabilisation of the parent structure caused by O dimerization. This microscopic understanding leads to better design of Li-rich high Ni-content cathodes with higher capacity and minimal degradation.